Automatic test equipment is commonly used by electronics device manufacturers for detecting manufacturing defects. For example, automatic test equipment allows semiconductor device manufacturers to test, on a large volume basis, the functionality of each device sold in the marketplace. The tester drives signals to and detects signals from a device-under-test (DUT) and evaluates the detected results to expected values. Timing jitter degrades electrical systems and the push to higher data rates and lower logic swings has increased interest and necessity for the measurement and characterization of jitter.
Jitter is a key performance factor in high-speed data communications. Jitter is defined as the misalignment of the significant edges in a sequence of data bits from their ideal positions. Misalignments can result in data errors. Tracking these errors over an extended period of time determines system stability. Jitter can be due to deterministic and random phenomena. Determining the level of these jitter components guides design improvement.
Jitter measurement techniques typically have the ability to measure the timing of significant edges in a data stream. For example, oscilloscopes and digitizers have been used to measure the voltage of a signal at fixed time intervals and to analyze this data to determine edge times. Other examples are time interval analyzers and time stampers. These devices directly measure edge times or the time between a pair of edges. In yet another example, asynchronous strobing comparator techniques are used to measure whether a signal is above or below a threshold at fixed time intervals. Asynchronous strobing comparator techniques use stochastic mathematical techniques on the measurement data to determine characteristics of the edge times. Two general methods are used to establishing a fixed time interval with the asynchronous strobing (or sampling) techniques. The methods are “in-order” and “out-of-order” strobing. Shortcomings exist for both the in-order and out of order strobing techniques. For example, in-order strobing has low noise immunity and long acquisition times and out of order strobing is sensitive to frequency errors, requires a complex setup for measurement and provides limited ability to analyze the frequency characteristics of the signal.
Desired information with regard to frequency characteristics of a signal is the measurement of jitter as a function of frequency. Oscilloscopes, digitizers, time interval analyzers, and time stampers provide a method to measure jitter as a function of frequency, but they are relatively expensive. In-order sampling asynchronous strobing techniques can measure jitter as a function of frequency. However, the long acquisition times required of in-order sampling limits the frequencies it can measure to relatively low frequencies that are of no interest. Current out-of-order asynchronous sampling techniques do not provide a method to measure jitter as a function of frequency.
For the reasons stated above, and for other reasons stated below which will become apparent to those skilled in the art upon reading and understanding the present specification, there is a need in the art for an asynchronous sampling method of measuring the jitter found in a waveform as a function frequency that is effective at measuring desired frequencies and is relatively inexpensive.